Pyridoxal-5′-phosphate as an oxygenase cofactor: Discovery of a carboxamide-forming, α-amino acid monooxygenase-decarboxylase

Significance Enzymes that activate dioxygen typically rely on flavins or transition metals as redox cofactors. We describe the discovery of the enzyme catalyst Cap15 that converts 5′-C-glycyluridine (GlyU), an unusual β–hydroxy-α-amino acid, to the modified nucleoside uridine-5′-carboxamide. In contrast to expectations, pyridoxal-5'-phosphate (PLP) is the sole cofactor that serves as the initiating reducing agent to activate dioxygen for incorporation into GlyU prior to decarboxylation. Thus, Cap15 is now classified as an O 2 - and PLP-dependent monooxygenase-decarboxylase. Also in contrast to expectations, phosphate and the (5′ S ,6′ R ) isomer of GlyU—and not (5′ S ,6′ S )-GlyU, the established pathway intermediate—are essential for Cap15 activity. In total, Cap15 utilizes an enzymatic strategy for oxygen incorporation during conversion of an α-amino acid to a carboxamide.